1. Field of the Invention
This invention in general relates to a fiber optical system for transmission of information from one location to another. In particular, this invention relates to a system for measuring one or more physical parameters or data and transmission thereof to evaluation circuitry, which circuitry is located remote from the location of the measurement. Still more particularly, this invention relates to an optical sensor system using at least one transducer for measuring a physical parameter, preferably a multitude of transducers for measuring various physical parameters, and fiber optical means for optical transmission of the sensed parameters to an evaluation circuitry. Such optical sensor system may be applied, for instance, in industrial facilities such as power plants or chemical plants, or in automobiles for the measurement and/or control of various functions.
2. Description of the Prior Art
In "Control Engineering", February 1979, pages 30-33, fiber optical sensors and transducers for measurement of physical parameters are disclosed. These transducers generate digital signals in accordance with the physical parameter being measured without using analog to digital (A/D) converters. The transducers require only light in order to become energized. The aforementioned sensors may be applied for measuring temperature, pressure, flow rate, and similar physical parameters. They are included in fiber optical transmission systems. When multimode fibers are used in these systems, the transmission schemes can be digital intensity modulation (binary, pulse width, frequency, etc.) wave length or color modulation, and color multiplexing.
In the measurement of physical parameters, it is generally desirable that the transducer directly produce digital signals, rather than require an electric A/D converter. Further, the transducer should require power only in the form of optical power. In addition, the design of the transducer and the optical transmission line should be such that the system can withstand hostile environments, including electromagnetic interference.
In "Control Engineering", supra, page 32, third column, and page 33, lower figures, a fiber optical sensor system is disclosed which works as a luminescent temperature transducer. The system includes a pulsed light source which emits light pulses into the first end of a fiber optical cable. The remote sensor is simply the expanded second end of the fiber optical cable, which end is coated with or is embedded in a phosphorescent material. Phosphorescence of this material occurs after each light pulse which has been received from the light source. A response or return light signal is emitted from the phosphorescent material in backward direction through the fiber optical cable. By means of a light coupler associated with the first end of the optical cable, the response signal is coupled to a photodetector. The electric output signal of the photodetector, that is the post-excitation return signal, is fed into a sample-hold or time-constant measurement device. Here, the decay time constant of the phosphorescent process is measured. The temperature sensor system provides a pulse width which is proportional to the decay time constant, and therefore, proportional to the temperature to which the phosphorescent material is exposed. This fiber optical sensor system is based on the measurement of a decreasing analog signal and therefore subject to disturbing influences. In addition, the costs for such a system may be high. Thus, there is a need for a fiber optical sensor system which is not only precise and reliable, but which is also inexpensive to manufacture. Such a system should have some sort of energy source on the measurement side so that active electric devices, such as transistors, may be energized. The system should derive the required power from optical signals transmitted through a fiber optical transmission line, and it should not require any additional sources of electric power at the location(s) of measurement.
A fiber optical sensor system of the kind just described appears to be particularly suited for use in automobiles. In conventional automotive applications, more than a dozen parameters are sensed at the automobile engine alone, and more than a dozen other parameters are measured at other locations of the automobile. A sensor system applied in a car should require only a minimum of cabling, should be extremely reliable, even under extreme environmental conditions like heat, cold, humidity, vibration, chemical and mechanical stress, and also under the usual wear and tear to which an automobile is subjected. A fiber optical transmission cable by nature would be little effected by all these factors and would exhibit the additional advantage that is not subject to electromagnetic interference, which may be generated by the ignition system of the car and by other devices. Therefore, in automobile applications an electro-optical sensor system may be used widely, if such a system could be manufactured inexpensively and if the accuracy required for the sensors can be obtained. This is not only true for automobile applications, but also for other applications with similar requirements, such as industrial applications.